Project Summary: Synaptic Reprogramming of Adult Neurons The nervous system is largely non-renewing. The adult brain, then, has two choices to protect against neural dysfunction, vigorously maintain existing circuitry and establish new connections when cells or synapses become lost. Despite the importance of these processes, we know almost nothing about the structural and molecular pathways that regulate them. To address this problem, our goal is to define how adult neurons encode synaptic integrity and rewire with new partners. Here, we propose a novel strategy to visualize and molecularly characterize single synapses in order to identify pathways that can modify adult synaptic connectivity. Using the highly ordered synapses of the mouse visual system, we have discovered that subsets of adult neurons can indeed alter their partner choice. This observation is critical for three reasons. First, it implies the existence of neuron-specific synaptic integrity pathways. Second, it indicates that neural maps may be flexible at given nodes, such that particular neuron subsets can structurally tune their connectivity. Third, it suggests that neural adult wiring can be selectively reprogrammed, representing potential opportunities for therapeutic intervention. We propose to test these predictions in two projects aimed at uncovering the neuron- specific regulators of adult synaptic reprogramming. In Project 1, we propose to elucidate the structural features of this remodeling and define the set of neurons that encode synaptic flexibility both in intact and defective circuits. These studies will use advanced 3-D nanoscopic optical imaging techniques that we are developing. In Project 2, we will identify novel synaptic reprogramming regulators using new strategies that enable single synapse transcriptional profiling from molecularly identified neuron subsets. In parallel, we will test the impact of these pathways on circuit function using both broad measures of circuit integrity as well as directed cellular recording. Results from these studies will identify new basic mechanisms that regulate adult synaptic integrity, define molecular targets for repairing nervous system injury and provide a foundation for the development of therapeutics aimed at maintaining brain health.